System and process for recording ERG, PERG and VEP multifocal electrofunctional responses in real-time

ABSTRACT

Process and system for determining the topography of bioelectric response signals of a visual system including a patient&#39;s retina, optical nerve or a projection thereof at the level of the central cortex, following visual stimulation through a surface arranged in front of the patient&#39;s eye. An image which comprises a plurality of cells is displayed as stimulation, each cell being activated or deactivated according to a corresponding digital time function represented by a cyclical succession of binary m-sequences of duration (T) formed from a plurality of activation symbols (N), each having a duration (Ts), the m-sequences of the various cells being obtained cyclically from a mother m-sequence. The process determines the total bioelectric response of the visual system, the response associated with each cell being determined by the total response of the visual system using a cross-correlation with a suitable translated version of a mother m-sequence. The calculation of the response of each cell is updated at the end of each symbol time (Ts), thereby making it possible to follow the evolution of the calculated response of each cell in real-time, without awaiting the end of an m-sequence.

FIELD OF THE INVENTION

The present invention relates generally to ophthalmology and, more particularly, to diagnosis of major ocular pathologies including, but not limited to, glaucoma, retinal anomalies and of sight, retinal degeneration of the retinal structure and retinal macular degeneration, as well as disorders of the optical nerve and the visual cortex.

BACKGROUND OF THE INVENTION

Conventional ophthalmic diagnostic techniques are based on recording bioelectric responses generated by the retina, the optical nerve and all cellular and nerve processes including the central visual cortex, as a consequence of a visual stimulus perceived by the patient. These bioelectric responses are recorded, through suitable electrodes arranged at the level of the conjunctival fornix, cornea, or near to the central visual cortex using surface electrodes (of both eyes in the case of binocular recording), from which a biopotential can be obtained. This represents a measurement of the integrity of the visual system (density of cones, rods and cells connected to them, gangliocytes, retinal cells, nerve fibres and visual cortex), or of possible alterations or destructive actions already caused by various pathologies.

With the current techniques (U.S. Pat. No. 4,846,567 July 1989, Sutter et. al.—E. B. Brown et al. Contrast luminance as parameter defining . . . Etc. Ophthalmic and physiological optic, Vol. 16 n. 1 January 1996 pp. 42-48—E. E. SUTTER ET AL.: “The Field Topography of ERG Components in Man” VISION RESEARCH, Bd. 32, Nr. 3, 1992, Seiten 433-446) it is possible to record a number n of biopotentials (responses generated by different locations, not only retinal) suitable for allowing the topography of the retina or of the central visual cortex.

Although widely used in current ophthalmologic diagnosis, this technique has some limitations, the first of which is the lack of control of the retinal and/or consequently cortical areas really stimulated, and the instant feedback of the result of the bioelectric responses recorded. In order to stimulate and consequently record the biopotential generated by the different areas, techniques have been developed including the one documented in E. E. SUTTER ET AL.: “The Field Topography of ERG Components in Man” VISION RESEARCH, Bd. 32, Nr. 3, 1992, Seiten 433-446, based upon stimulation through “m-sequences” and the subsequent decoding by means of cross-correlation between stimulated area and consequent biopotential (retinal or cortical reaction) recorded. However, it requires very long execution times and allows possible artifacts (biopotentials induced by loss of focus or movements of the patient's eyes) to be monitored but does not allow them to be quickly corrected. This can contaminate the subsequent result, as widely documented in Guideline for basic Multifocal Electroretinography (mfERG) Documenta Ophthalmologica n. 106 pp.105-115, 2003.

Further improvements of the current technique are disclosed in U.S. Pat. No. 6,022,107 August 2000 Ernst Kutschbach et. al., with which improvements are introduced such as the fragmentation of the m-sequence into steps lasting a shorter time that allow an intermediate evaluation point of the quality of the result, also allowing possible samples—judged to be unreliable thanks to a discarding automatism of the bioelectric responses exceeding predetermined size limits—to be repeated. Such a technique does, however, require waiting until the end of each cycle of each m-sequence for some repetitions, imposing long and pointless waiting times on the patient.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a system and an apparatus that further improves quality control, thereby eliminating uncertainties in measurement due to the intrinsic variability of the result itself in conventional clinical practice.

This purpose is accomplished with the process for recording ERG, PERG, VEP and multifocal electrofunctional responses in real time according to the present invention, the essential characteristics of which are defined in the first of the attached claims. A system specifically adapted to carry out the process according to the invention has the essential characteristics defined in claim 7.

BRIEF DESCRIPTION OF THE DRAWINGS

A specific, illustrative system and process for recording ERG, PERG, VEP and multifocal electrofunctional responses in real time, according to the present invention, is described below with reference to the accompanying drawings, in which:

FIGS. 1 a and 1 b show an arrangement of electrodes, on the face and head of a patient, for recording retinal and cortical biopotential, according to one aspect of the present invention;

FIG. 2 illustrates schematically hardware of a system, according to one aspect of the present invention;

FIG. 3 shows schematically a hardware system, according to another aspect of the present invention;

FIGS. 4 a-4 h show respective examples of pattern stimuli according to the process of the present invention;

FIG. 5 is an image representing a display that can be obtained by the medical operator with the system set forth in FIG. 3;

FIG. 6 illustrates pseudo-orthogonal m-sequences generated by cyclic translation of a mother m-sequence, in accordance with one aspect of the present invention;

FIG. 7 shows a time-scale of a series of m-sequences, according to another aspect of the present invention;

FIG. 8 is a diagram showing cross-correlation of the overall response of each part highlighted in the time-scale of FIG. 7 with the m-sequence associated with the q-th cell to obtain the current response of the q-th cell due to the m-sequence just ended;

FIG. 9 illustrates a time-scale, according to a further aspect of the present invention, where k is equal to a whole number of time periods Ts that follow the end of the previous m-sequence after which an update is desired as to the wave-forms for the responses of the various cells, and wherein a part of overall response of length N*Ts that goes from the moment T_(refresh) at which an update is desired, up to T_(refresh)−N*Ts; and

FIG. 10 is a diagram in which, for each cell, the sequences to be cross-correlated with the overall response acquired in the time period from T_(refresh) up to T_(refresh)−N*Ts are represented, where k is equal to 4 and the set of m-sequences used is the one previously represented.

The same numerals are used throughout the drawing figures to designate similar elements. Still other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and, more particularly, to FIGS. 1-5, there is shown generally a specific, illustrative system and process for recording ERG, PERG and VEP multifocal electrofunctional responses in real-time, according to various aspects of the present invention. In one embodiment, illustrated generally in FIGS. 1 a, 1 b, 2 and from 4 a-4 h, a configuration of electrodes is arranged on the face of a patient to be examined, as in conventional ERG, PERG and VEP recording techniques. Specifically, in case of retinal biopotential (FIG. 1 a), an active exploring electrode is inserted inside the lower conjunctival fornix (positions 1) or on the corneal surface of each eye, the electrode coming into contact with the eyeball or also at the level of the central visual cortex using surface electrodes, so as to pick up a bioelectric signal generated by the stimulated areas. Preferably, in order to record the retinal biopotential, the electrode is of the HK LOOP type, which comprises a very thin silver wire coated with a sheath constructed, e.g., of TEFLON, on which some incisions are made to allow contact with the patient's eyeball.

This type of electrode does not interfere with the patient's vision, also allowing him to comfortably blink, these being characteristics that are not found, for example, in the use of common corneal electrodes. Moreover, a better contact impedance between the electrode and the patient, and therefore a more reliable contact is obtained than cutaneous electrodes, possible refractive problems connected to the use of corneal electrodes is avoided, and it is possible to record the bioelectric signal with a very high signal/noise ratio, for which reason the time taken for an examination is substantially reduced (the number of samples to be taken is reduced).

A reference electrode is positioned, following suitable cleaning of the skin, at the level of the outer temporal corner of each eye (position 2, FIG. 1 a), and comprises a silver or silver chloride disc to which a special electroconductive paste is added so as to assist the contact with the patient's skin. A common electrode, of the same type as the reference electrode, is finally arranged at the center of the patient's forehead (position 3). In case of cortical detection, again as an example, the positioning of the explorating active electrode, reference electrode, and common electrode, follows the configuration respectively indicated by the positions 11, 12 and 13 of FIG. 1 b.

The patient, with the aforementioned electrodes already applied, is positioned a distance of about 30 cm from an apparatus for performing the examination, with the chin resting on an adjustable chin-rest. The apparatus, shown in FIG. 2, comprises a personal computer 4 with a software system suitable for simultaneously managing a generation of a sequence of visual stimuli shown in a screen 5 arranged at the height of the patient's eyes, and for carrying out a consequent recording of the biopotential. In accordance with what has already been disclosed in conventional apparatuses, the average brightness of the screen is calibrated according to the type of examination according to preselected values.

The recording of the biopotential takes place, in the illustrated embodiment, through the use of a differential amplifier 6 connected both to the personal computer 4 and to all the electrodes 1, 2, 3 (or else 11, 12, 13). Such a differential amplifier is characterized by a passband of between 0.1 Hz and 3000 Hz and an amplification factor of between 50,000 and 500,000 V/v. A 16 bit A/D converter (not shown) digitizes the signal, all according to the known conventional applications.

The software residing in the personal computer, above and beyond allowing the recorded biopotential to be processed, this processing being printable with a printer 7, as stated controls the generation of pattern stimuli presented on the screen 5. Said stimuli are configured on the screen as shown, for example, in FIGS. 4 a to 4 d. The patient is shown a sequence of cell patterns, i.e. a group of visual stimuli having various spatial and temporal forms. In synchrony with the stimulus, the system acquires the sampled signal to be processed (bioelectric response associated with the presentation of a visual stimulus) from the electrodes suitably positioned on the patient. Each cell on the screen is modulated based upon a corresponding binary m-sequence, in the way explained hereafter.

The ultimate goal of the examination is to associate a bioelectric response with every cell belonging to the stimulation pattern, i.e. to the retinal portion or cortical projection associated with such a cell. For MF-ERG examination, the term “activated” must be interpreted as “lit” and “deactivated” must be interpreted as “not lit” (FIG. 4 a, in which some cells are lit, others not). For MF PERG and VEP examinations, on the other hand, the word “activated” must be interpreted as placed in the “normal pattern condition”, and “deactivated” must be interpreted as placed in the “inverse pattern condition” (FIG. 4 c, in which some cells have a normal pattern, other inverse pattern).

As represented by FIGS. 4 b and 4 d, it is also possible to modify the characteristics of the images, for example color, brightness, contrast but also shape, spatial size, temporal alternation, distortion, visual angle subtended. In this way, the n retinal areas or relative cortical areas that result from the characteristic of the type of multifocal stimulus used for the stimulation of the patient can be stimulated independently.

Furthermore, FIGS. 4 e and 4 f show a different set of stimuli suitable for selectively stimulating visual areas or retinal and cortical receptive fields, both horizontal and vertical. FIG. 4 g represents a different stimulus set made up of a plurality of cells (hexagons) suitable for stimulating a different amount of visual areas or retinal and cortical receptive fields. Finally, FIG. 4 h shows a different set of images made up of a plurality of cells divided into sectors of concentric rings further divided into white-black elements or contrasting colors suitable for stimulating visual areas or retinal and cortical receptive fields.

Returning specifically to the control of the stimulation, the recording of the biopotential and, therefore, the measurement of the retinal or cortical biopotential, specifically for the n areas stimulated, takes place as follows.

The aforementioned m-sequence is a sequence of symbols 1 and −1 of length N=2^(s)−1,s being a positive whole number. If the m-sequence is N symbols long it is possible to manage a pattern of M cells (M<=N). Each cell is associated with an m-sequence N symbols long that can be obtained by cyclically translating a mother m-sequence. The various m-sequences generated by cyclic translation by a mother m-sequence are pseudo-orthogonal, according to the example diagram shown is FIG. 6 (s=3, N=7).

In the time-scale, illustrated generally in FIG. 7, Ts is the period of a symbol time of the m-sequence, i.e. of a 1 or −1. T is the period of an entire m-sequence, i.e. the period of a symbol time multiplied by the length of the m-sequence N. Be it assumed that k-periods Ts have passed from the start of the m-sequence, if the q-th m-sequence associated with the q-th cell contains a 1 in the k-th position, the cell of the pattern on the screen shall be activated, otherwise it will be deactivated.

The time period between two stimuli is selected so as to be great enough to take up the entire bioelectric response in such a time.

At the end of each m-sequence, i.e. after each period of length N*Ts, it is possible to calculate the response of every cell based upon the data gathered during the execution time of the m-sequence just ended. Assuming that one is interested in the response of the q-th cell, with which the q-th m-sequence obtainable through a cyclic translation of q symbols of the mother m-sequence is associated. As set forth in FIG. 8, the data of the overall response of each part highlighted in the time-scale shown in FIG. 7 can be cross-correlated with the m-sequence associated with the q-th cell, to obtain the current response of the q-th cell due to the m-sequence just ended. If a time period equal to a whole number n of m-sequences has passed, it is possible to calculate the average response of each cell using the responses of each cell derived from the data collected during the first, second, up to the n-th stimulation m-sequence.

According to a specific and characteristic aspect of the present invention, contrary to what was done previously, the calculation of the response of every cell is updated at the end of every symbol time equal to T/N. In this way, it is possible to follow the evolution of the calculated response of each cell in real time without having to wait for the end of an m-sequence. Consequently, there is a substantial benefit in terms of saving time and also immediately displaying the effectiveness of the result and possible errors.

The above can be explained in detail in the following example, shown in FIG. 9. Be it assumed that k is equal to a whole number of time periods Ts that follow the end of the previous m-sequence after which it is wished to obtain an update of the wave-forms of the responses of the various cells. Be it then considered the part of overall response of length N*Ts that goes from the moment T_(refresh) at which an update is wanted, up to T_(refresh)−N*Ts.

Supposing that one is interested in the response of the q-th cell with which at the beginning the q-th m-sequence obtainable through a cyclic translation of q symbols of the mother m-sequence is associated, the data extracted from the overall response (from T_(refresh) up to T_(refresh)−N*Ts =T_(refresh)−T) must be cross-correlated not with the m-sequence associated with the q-th cell but with a version thereof cyclically delayed by k symbols to obtain the current response of said q-th cell due to the N most recent stimuli (generated in a time period T). See the example in the diagram set forth in FIG. 10 in which for each cell the sequences to be cross-correlated with the overall response acquired in the time period from T_(refresh) up to T_(refresh)−N*Ts are represented. In this example, k is equal to 4 and the set of m-sequences used is the one previously represented.

This partial result can be used to calculate the average response associated with the q-th cell in combination with the partial results calculated previously for this same cell.

The calculated wave-forms associated with the various cells can be directly interpreted or else associated with a pattern made up of luminous, isoluminant or gray-scale chromatic stimuli that mirrors the matrix of the stimuli.

To sum up, the displaying in real time of the results allows the operator or automatically allows the processor to:

-   -   monitor possible loss of attention by the patient or possible         artifacts that can contaminate and jeopardize the outcome of the         examination;     -   interrupt the acquisition cycle of the biopotential at any         moment;     -   repeat only the stimulus part where an artifact of the         biopotential associated with the responses generated by specific         cells has been identified.

The time taken for the examination is thus significantly reduced with the result of optimizing the attention, learning and fatigue capability of the patient, having the advantage, as instantaneous evaluation element, of an intrinsic coefficient of variation (CV) and a standard error of the mean (SEM) of the result obtained in real time, and at the same time the qualitative analysis of the result.

Regarding this last point, it is clear that, according to the selected analysis strategy, during the acquisition process the bioelectric responses currently calculated

$\overset{\_}{A} = {\sum\limits_{0}^{N_{c} - 1}{{}_{}^{}{}_{}^{}}}$ $\sigma_{A} = \sqrt{\frac{1}{N_{c} - 1}{\sum\limits_{0}^{N_{c} - 1}{\cdot \left( {A_{i} - \overset{\_}{A}} \right)^{2}}}}$

could differ from the corresponding previous ones. The coefficient of variation (CV) provides as instantaneous information, the standard deviation of the measurements, i.e. the root mean square σ_(A) of the responses calculated in successive time periods purged of the mean value Ā.

The standard error of the mean (SEM) equal to the standard deviation of the samples measured divided by the root of the number Nc of samples measured, on the other hand, is used to evaluate the reliability of the measurement.

${C\; V} = \frac{\sigma_{A}}{\overset{\_}{A}}$ ${S\; E\; M} = \frac{\sigma_{A}}{\sqrt{N_{c}}}$

It should be noted that such a parameter cannot be clinically tested but provides the clinic operator with data on how much the phenomenon has been correctly characterized in statistical terms.

Advantageously, it is possible to compare the result obtained from the patient examined with a normative database derived from extensive statistics collected on samples of normal subjects, capable of providing an indication of the deviation of the subject compared to the average of the normal subjects in relation to age. For example, a statistical test carried out on 154 healthy eyes was performed in order to define the normal ranges for the parameters of size and latency of the bioelectric responses correlated with the retinal surface stimulated by the various electrofunctional examinations. The statistical analysis was carried out on a population of normal subjects and in order to establish the existence and the correlation with the age of the patient. The examinations considered belong to a variegated population of patients from a minimum age of 12 years old up to a maximum of 87. The results obtained show the dependence of the size and latency upon the age of the subject examined. Such normative data, or other analogous data, after having been evaluated from the statistical point of view, can be inserted as a physiological parameter of “normality” into the apparatus, in order to make the user able to quickly and effectively consult it.

The present invention thus makes it possible to obtain a result immune from intrinsic factors of variability of the conventional technique like for example problems linked to the loss of focus or eye movements. The apparatus described above as an example allows the examination of a single eye or of both eyes of the patient simultaneously. According to a different embodiment, schematized by FIG. 3 (in which part identical or corresponding to those already described for the first embodiment are indicated with the same reference numerals), on the other hand, it is only necessary to examine one eye at a time. Moreover, such a solution ensures even greater precision of the results, avoiding the contamination of the biopotential with possible unreliable recordings.

In particular, the apparatus of FIG. 3 differs from the previous one in that a mirror or prism for optical beam separation, also known as beam splitter, of a per se known type and indicated at 8, is arranged between the eye of the patient and the pattern stimulator (CRT, CCD, LED, OLED, PLASMA). The beam splitter, arranged in a suitable position on the optical path, allows the patient to be shown a visual stimulus and at the same time allows the operator to be shown the image of the relevant portion of the patient's eye.

The medical operator, symbolized in the figures by the eye 9, directly or by means of a camera in turn connected to a monitor, can thus observe the retina base of the patient during the course of the examination thanks to an ophthalmoscope (not shown), ensuring that the stimulus is projected exactly at the centre of the fovea.

Therefore, he is able to instantly make corrections, interrupting and then continuing the stimulus associated with the response of the stimulated area (this thanks to the real time control according to the present invention), as a consequence of a loss of focus of the patient on the stimulus, for example due to an eye movement, blinking or attention difficulties in following the examination itself. The operator can also temporarily interrupt the recording in the case of the patient experiencing slight momentary difficulties as the examination progresses. The exact projection of the stimulus in the foveal area thus allows the occurrence of unreliable measurements caused by the patient having difficulties during the examination to be minimized.

In a particularly advantageous embodiment, the beam splitter, the ophthalmoscope, the real time photography of the fundus oculi and the focusing of the stimulus on the desired retinal areas, the system for collimation, pointing and correcting the refractive error or ametropia of the patient, and a suitable optical system for projecting the pattern are stably integrated in a single optical-mechanical-electronic assembly, so that the view field of the ophthalmoscope and the projection field of the pattern exactly and unequivocally match up. Indeed, FIG. 5 provides an idea of the matching up of the response generated by each individual cell with the image of the fundus oculi of the patient examined.

With these solutions, therefore, also the conventional comparison between the response obtained from the patient with the normality data correlated with the age of the subject and obtainable from clinical testing, can be carried out much more precisely and reliably, which in any case allows the coefficient of variability connected to the current technique to be substantially reduced. The result of the diagnosis is really independent of the operator's ability and the patient's will, as well as of all other factors and sources of variability described above.

In both embodiments, the possibility of creating an archive of the results obtained for subsequent diagnosis makes it possible to monitor the progress of the pathology over time, and the possible effectiveness of a pharmacological treatment.

Various modifications and alterations may be appreciated based on a review of this disclosure. These changes and additions are intended to be within the scope and spirit of the invention as defined by the following claims. 

1. A process for determining the topography of bioelectric response signals of a visual system of a patient, the patient's retina, optical nerve or a projection thereof being at the level of the central cortex, following visual stimulation through a surface arranged in front of the patient's eye, wherein an image which comprises a plurality of cells is displayed as stimulation, each cell being activated or deactivated according to a corresponding digital time function represented by a cyclical succession of binary m-sequences of duration (T) formed by a plurality of activation symbols (N), each having a duration (Ts), the m-sequences of the various cells being obtained cyclically from a mother m-sequence, the process determining the total bioelectric response of the visual system, the response associated with each cell being determined by the total response of the visual system using a cross-correlation with a suitable translated version of a mother m-sequence, wherein calculation of the response of each cell is updated at the end of each symbol time (Ts), thereby making it possible to follow the evolution of the calculated response of each cell in real time, without awaiting the end of an m-sequence.
 2. The process set forth in claim 1, wherein updating is accomplished, after a certain number (k) of symbol times (Ts) to obtain the response of a q-th cell associated with an m-sequence translated by a number (q) of symbols from the mother m-sequence, by cross-correlating the data extracted from the overall response not with the translated m-sequence, but with a version thereof cyclically delayed by the number (k) to obtain the current response of the q-th cell due to a number (N) of most recent stimuli.
 3. The process set forth in claim 1, wherein calculated wave-forms associated with the various cells are either directly interpreted or are associated with a pattern comprising luminous, isoluminant or gray-scale chromatic stimuli that mirrors the matrix of the stimuli.
 4. The process set forth in claim 1, wherein the accuracy of the acquisition step is evaluated based on an intrinsic coefficient of variation (CV) and a standard error of the mean (SEM) of the result obtained in real-time.
 5. The process set forth in claim 4, wherein the intrinsic coefficient of variation (CV) and the standard error of the mean (SEM) are calculated in accordance with ${C\; V} = {{\frac{\sigma_{A}}{\overset{\_}{A}}\mspace{14mu} {and}\mspace{14mu} S\; E\; M} = {\frac{\sigma_{A}}{\sqrt{N_{c}}}.}}$
 6. The process set forth in claim 1, wherein the result obtained by the patient examined is compared with a normative database that includes results obtained on normal subjects, to indicate the deviation of the subject compared to the average of the normal subjects in relation to age.
 7. A system for determining the topography of bioelectric response signals of a visual system of a patient including the patient's retina, optical nerve or a projection thereof at the level of the central cortex, following visual stimulation, the system comprising a display arranged in front of the patient's eye, a processor connected to the display for showing an image, as stimulation, including a plurality of cells, each cell being activated or deactivated according to a corresponding digital time function represented by a cyclical succession of binary m-sequences of duration (T) formed by a plurality of activation symbols (N), each having a duration (Ts), the m-sequences of the various cells being obtained cyclically from a mother m-sequence, the system further comprising a sensor and amplifier for determining the total bioelectric response of the visual system, and recording the response on the processor, such that the response associated with each cell is determined by the total response of the visual system using a cross-correlation with a suitable translated version of a mother m-sequence, wherein processor updates the calculation of the response of each cell at the end of every symbol time (Ts), thereby making it possible to follow the evolution of the calculated response of each cell in real-time, without having to wait for the end of an m-sequence.
 8. The system set forth in claim 7, wherein the processor performs the updating operation, after a selected number (k) of symbol times (Ts) to obtain the response of a q-th cell associated with an m-sequence translated by a number (q) of symbols from the mother m-sequence, by cross-correlating the data extracted from the overall response not with the translated m-sequence, but with a version thereof cyclically delayed by the number (k) to obtain the current response of the q-th cell due to a number (N) of most recent stimuli.
 9. The system set forth in claim 7, wherein the processor interprets either the calculated wave-forms associated with the various cells directly or those associated with a pattern made up of luminous, isoluminant or gray-scale chromatic stimuli that mirrors the matrix of the stimuli.
 10. The system set forth in claim 7, wherein the processor evaluates the accuracy of the acquisition step based on an intrinsic coefficient of variation (CV) and a standard error of the mean (SEM) of the result obtained in real-time.
 11. The system set forth in claim 10, wherein the intrinsic coefficient of variation (CV) and the standard error of the mean (SEM) are calculated in accordance with ${C\; V} = {{\frac{\sigma_{A}}{\overset{\_}{A}}\mspace{14mu} {and}\mspace{14mu} S\; E\; M} = {\frac{\sigma_{A}}{\sqrt{N_{c}}}.}}$
 12. The system set forth in claim 7, wherein the processor comprises a storage member of a normative database that includes results obtained on normal subjects, the result obtained from the patient examined being compared to the database, so as to indicate deviation of the subject compared to the average of normal subjects relative to age.
 13. The system set forth in claim 7, wherein between the patient and the display, an optical beam separator is arranged, suitable for allowing the patient to observe the stimulation reflected by the divider instead of directly from the display, thus enabling a medical operator, for control purposes, to observe the retina base of the patient during the course of examination.
 14. The system set forth in claim 13, wherein the optical beam separator, the display and ophthalmoscopic member for observing the retina base of the patient during the course of examination, are stably integrated in a single optical-mechanical assembly.
 15. A program-controlled apparatus for determining the topography of bioelectric response signals of a visual system of a patient, the patient's retina, optical nerve or a projection thereof being at the level of the central cortex, following visual stimulation through a surface arranged in front of the patient's eye, which performs the steps of: (i) displaying an image comprising a plurality of cells as stimulation, each cell being activated or deactivated according to a corresponding digital time function represented by a cyclical succession of binary m-sequences of a first selected duration formed from a plurality of activation symbols, each having a second selected duration, the m-sequences of the various cells being obtained cyclically from a mother m-sequence; (ii) determining the total bioelectric response of the visual system, the response associated with each cell being determined by the total response of the visual system by a cross-correlation with a suitable translated version of a mother m-sequence, wherein calculation of the response of each cell is updated at the end of each symbol time (Ts), thereby making it possible to follow the evolution of the calculated response of each cell in real-time, without awaiting the end of an m-sequence.
 16. The apparatus set forth in claim 15, wherein the process is stored in a memory support device.
 17. The apparatus set forth in claim 15, wherein the process is stored in the processor of the system. 